This invention concerns methods and apparatus for the evaluation of surface roughness characteristics by non-contacting or optical means.
The conventional approach in the determination of surface roughness characteristics for industrial applications is by use of a Profilometer surface roughness gauge which in mechanical stylus is traversed across the surface to be measured with an electrical transducer reading the micromovement of the stylus. As the peaks and valleys are traversed, the readings obtained from this apparatus produce a standardized number value corresponding to the roughness of the surface measured. This is the root mean square or .sigma. standard unit to specify surface roughness.
The major application of surface roughness measuring equipment is in the manufacture of machined parts, since for many moving parts such as compressor pump shafts supported within bearings, surface roughness is the crucial factor in determining the life of the parts in service.
The conventional approach utilizing a stylus gauge has many disadvantages in the context of industrial or manufacturing applications. These include the delicate nature of the gauge producing unsatisfactory performance under rough service with many parts being gauged; the output signal does not yield a single signal output corresponding to surface roughness, but rather a continuously varying output as each surface irregularity is traversed, and accordingly an average dwell must be derived in order to provide a surface roughness indicator signal. In addition, the precise nature of the set up and alignment of the stylus and the test surface precludes good repeatability of readings obtainable in the factory environment where, typically with heavy building, vibrations are present.
The Profilometer surface roughness measuring instruments also require a relatively high degree of skill and it is quite time consuming to obtain measurements.
For this reason, the usual approach in achieving quality control in the manufacture of parts is to sample parts within a production lot, with scrapping or reworking of the entire lot, if the surface roughness of the sample parts do not measure up to the proper standards. This is inefficient and wasteful since an error in set up or procedures of the machining or grinding operation may be allowed to continue for an entire lot before correction of the problem. In addition, the good parts within a lot are either scrapped or required to be reworked.
Accordingly, it has heretofore been proposed in many prior art patents to provide a noncontacting or optical surface roughness measurement device and method which would obviate many or most of the above-described deficiencies. However, heretofore no such optical or noncontacting instrument has successfully provided an alternative to the mechanical approach for use in industrial applications.
The usual prior art approach has been to measure the ratio of specular to diffuse light level reflected from the surface which roughness is to be determined, upon illuminating the surface with an incident beam from a light source. The principle relied on is that the rougher the surface, the proportion of the light reflected specularly decreases as the surface becomes rougher.
The use of the proportion of specular to diffuse reflection rather than simply sensing the change in specular reflection intensity eliminates the effect of surface coloration and illuminating beam intensity variations.
While this is a theoretically adequate principle, as a general proposition, the need to sense the diffuse component of reflected light with its relatively low intensity, has precluded reliable performance of such devices. In addition, many machined parts have a characteristic lay to the roughness, i.e., grinding marks in a certain direction, which greatly affect the character of the diffused reflected light.
Many of these patents accordingly scan the surface at various oblique angles to develop a diffuse specular analysis.
Examples of various approaches using this basic principle are found in U.S. Pat. Nos. 3,804,521; 3,229,564; 3,771,880; 3,746,869; 3,591,291 and 3,850,526.
It has also been proposed to measure surface roughness based on the intensity of the specular component alone by the use of a constant intensity light source of a known wavelength, i.e., a laser source. This is described in an article in Ceramic Bulletin, "A Laser Reflectometer for Ceramic Surface Diagnostics", Volume 52, No. 2 (1973) pages 191 to 194.
This approach ignores the effect of variations in source intensity, however, which would lead to inaccurate results. In addition, it depends on the constant overall reflectivity of the surface as due to coloration rather than surface roughness which further would contribute to errors in such systems.
As noted above, the most common application of surface roughness measurement for industrial applications is in bearing surfaces which is most usually shafts having a circular cross section. It would be highly advantageous if any such apparatus and method could be directly applicable to such round shafts such that the device or method could be readily adapted to 100% inspection or combined with machine controls so as to produce an in-process gauging of parts.
It is accordingly an object of the present invention to provide a noncontacting optical method and apparatus for determining surface roughness which will produce a highly reliable evaluation of the roughness of surfaces at workpieces inspected.
It is another object of the present invention to provide an optical noncontaining surface roughness apparatus and method which does not rely on the detection of the diffuse component of the light reflected from the surface.
It is yet another object of the present invention to provide such an optical method and apparatus which is not affected by variations in the surface color, the character of the surface irregularities or in the intensity of the source utilized to illuminate the surface.
It is yet another object of the present invention to provide such an optical method and apparatus which is particularly adapted to rounded surfaces and which quickly and easily produces a surface roughness evaluation which is highly reliable and requires minimal skill to accurately interpret.